First, with reference to FIG. 9, a description will be given to an example of a conventional brushless motor mounted on a general full automatic washing machine or a drum washing machine (hereinafter, simply referred to as a washing machine). FIG. 9 is a block diagram of conventional washing machine 600 provided with brushless motor 610 to which a rotation speed (hereinafter, simply referred to as a speed) instruction is input and a speed of which can be controlled according to the speed instruction.
Brushless motor 610 is configured with analogue IC type motor control circuit 620 having no arithmetic processing function, inverter circuit 20, temperature sensor 21 which detects a temperature of a power element incorporated in inverter circuit 20, current detecting element 22 which detects a power supply current Iz, motor 50, and position sensor 57 which detects a rotor position. Motor control circuit 620 has speed controller 621 which generates a PWM (pulse-width modulation) duty signal based on a speed instruction signal Sz informed from body-side micro controller (hereinafter, referred to as a microcomputer) 670, a signal of a reference triangular wave (not shown), and the like.
Washing machine body 601 is configured with body-side microcomputer 670, power supply unit 80, brushless motor 610, washing tub 90, various sensors 130, operation unit 112, and display unit 113. Body-side microcomputer 670 is configured with body control function unit 71 and motor speed instruction unit 72. Body control function unit 71 performs control and causes washing tub 90 to arbitrarily operate, by using signals of various sensors 130 and a speed signal having been subjected to signal processing on brushless motor 610. Motor speed instruction unit 72 generates a speed instruction to appropriately operate brushless motor 610 and outputs the speed instruction to brushless motor 610 as the speed instruction signal Sz such that washing tub 90 performs an arbitrary operation.
Here, a logic of an operation of the motor will be described. FIG. 10 is a diagram showing an operation example of brushless motor 610 in one cycle of washing, and FIG. 11 is an enlarged view of a part, of FIG. 10, at the time of start-up. With reference to FIG. 10 and FIG. 11, the horizontal axes represent time t, and the waveforms represent, from top to bottom, signal waveform Sz(t) of the speed instruction signal Sz, actual voltage waveform Vz(t) of the signal waveform Sz(t), current waveform Iz(t) of the power supply current Iz, rotation speed RTz(t), and temperature change Tz(t) of a power element temperature Tz.
According to an operation mode selected by operation unit 112, in order to repeat operation/stop of the washing tub, the speed instruction signal Sz is output, from body-side microcomputer 670 to motor control circuit 620, as the signal waveform Sz(t) in a pulse shape having rise and fall as shown in FIG. 10 and FIG. 11. Here, the voltage of the speed instruction signal Sz corresponds to the instructed speed. Note that there are various impedance components from body-side microcomputer 670 to motor control circuit 620. With this configuration, the actual speed instruction signal Sz is input to motor control circuit 620, having an analogue signal waveform which has a time constant (around 200 milliseconds, for example) at the time of rising and falling as represented by voltage waveform Vz(t).
Motor control circuit 620 converts such a speed instruction signal Sz into a PWM duty signal of about 10 kHz to 20 kHz to drive inverter circuit 20. By this operation, inverter circuit 20 applies a voltage depending on the PWM duty signal to a winding wire of motor 50, so that brushless motor 610 operates.
As a result, a trajectory in which peak values of the current flowing through a power supply line are recorded shows a waveform represented by the current waveform Iz(t) of the power supply current Iz. In a period ranging from about a few milliseconds to a few hundred milliseconds after start-up, the rotation speed RTz(t) of the motor is relatively small, and a counter-electromotive force generated in the winding wire is accordingly small. Thus, almost all the applied voltage is applied to the winding wire, and the current peaks reach a current limit value Ip1 as shown in FIG. 11 (this state is referred to as an over-load state).
Here, an operation of limiting current will be described. The power supply current Iz is detected as a voltage signal by current detecting element 22 and is compared with a reference voltage corresponding to the current limit value. Then, if the voltage signal is equal to or greater than the reference voltage value, an operation of limiting current is performed. For example, in a method for limiting current, overcurrent is limited by repeating an operation in which switching according to the PWM duty signal of the power element in inverter circuit 20 is once turned off and the switching is restarted at the timing of the next duty instruction.
In this operation, the rotation speed of the motor rises toward a predetermined rotation speed shown in the rotation speed RTz(t). After the start-up, the rotation speed is gradually getting close to the predetermined rotation speed, and the peaks of the power supply current Iz are also gradually getting close to the current limit value Iq. Note that the above-mentioned current limit value Ip1 is a current limit value Ip1 in an over-load state (at the time of start-up), and the current limit value Iq is here a current limit value Iq for a steady load state.
Here, the current limit value Ip1 is set as described below. First, a protection operation temperature is defined by a package temperature at which a junction temperature of the power element does not exceed the maximum allowable junction temperature Tjmax, which is the allowable maximum value on the premise that the brushless motor generally performs a continuous operation. Here, as the current limit value Ip1, the maximum value of a current allowable value at the protection operation temperature is set. Further, the current limit value Iq is the maximum value of a current enough to generate torque necessary for the motor to rotate at a predetermined rotation speed. Note that, in the following description, the current limit value Ip1 is appropriately referred to as a first current limit value Ip1.
The power element temperature Tz, which is the temperature of the power element (package temperature) incorporated in inverter circuit 20, rises during an operation (washing operation) instruction and falls during a stop instruction as shown by the temperature change Tz(t). Typically, the temperature of the power element keeps rising as a whole in one cycle of washing while repeating rise and fall.
FIG. 12 is a diagram showing an example of a rising curve of the power element temperature Tz at the time of load operation of a real machine. The temperature rise of a real machine is generally checked after repetition of three cycles of washing, rinsing, and dewatering. Usually, of the components constituting a washing machine, the temperature rise is largest in the power element of the motor. For this reason, the body control is designed such that the temperature of the power element will not exceed the protection operation temperature after repetition of three cycles.
However, there is a possibility that an unexpected over-load operation or an abnormal usage which may lead to a motor lock can be performed during an actual usage of a washing machine, and in such a case, the temperature of the power element abnormally rises. To address this issue, a technique is conventionally proposed in which some measures are taken to prevent the junction temperature from exceeding the maximum allowable junction temperature Tjmax and the power element from getting broken. In an example of the measures, the protection operation temperature is previously set on the motor side, and if the temperature of the power element reaches the temperature, a protection operation is performed to prevent the power element from getting broken (for example, see PTLs 1 to 3).
FIG. 13 is a diagram showing a more specific operation example of the above protection operation. In the operation example shown in FIG. 13, if the power element temperature Tz exceeds the protection operation temperature Tp, the voltage of the speed instruction signal Sz is set to an OFF level to stop the drive of inverter circuit 20. By the above operation, the application of a voltage to the motor is terminated, and the flow of current is thus stopped, so that the motor stops turning, and the washing tub stops. Then, when the temperature becomes lower than a protection release temperature, the operation is restarted.
FIG. 14 is a diagram showing still another specific operation example of the above protection operation. The operation example shown in FIG. 14 shows a method in which the current limit value is lowered, and the adjusted current limit value Ip1′ is set so as to satisfy the relationship Ip1>Ip1′>Iq. In the above method, the power element temperature Tz and the current limit value are determined unambiguously by an analogue circuit, and the current limit value increases and decreases passively according to the power element temperature Tz. Further, because the current limit value Ip1′ satisfies the relationship Ip1′<Ip1, the current at the time of start-up becomes lower, and the torque generated by the motor is accordingly becomes lower, so that it takes a longer time to reach a predetermined rotation speed.
FIG. 15 is a diagram showing a still another operation example of the above protection operation. The operation example shown in FIG. 15 shows a case that the current limit value Ip1′ is set to satisfy the relationship Ip1′<Iq. In this case, because the current limit value Ip1′ is lower than the current limit value Iq which can generate enough torque to rotate at the predetermined rotation speed, the rotation speed cannot reach the predetermined rotation speed.
As described above, the conventional brushless motor in a washing machine has a protection function in which the rise in the temperature of the power element causes the motor to stop or in which the current limit value is lowered and the generated torque gets accordingly lower. Thus, there is a possibility that if the temperature of the power element becomes too high, the washing tub stops or the rotation speed does not reach the predetermined rotation speed.
Therefore, in order to prevent a change in motor output from affecting a washing time or a cleaning performance of laundry, consideration needs to be taken to the control of the washing machine body such that an overheat protection operation is not performed in repetition of operation and stop of the motor.
That is, in the case of an adjustment of time such as shortening an operation time or lengthening a stopping time or in the case of setting a predetermined rotation speed, settings need to be made with such a margin that the rise in the temperature of the power element never reaches the protection operation temperature even under the worst condition. In addition, in designing of controlling the washing machine body, design of the motor control is required to be made in order to prevent overheat of the power element.
As described above, the conventional analogue IC incorporated type brushless motor shown in FIG. 9 cannot determine the relationship between the power element temperature and the current limit value. That is, comparison is only performed between the temperature information of the power element and the reference voltage in order to change the current limit value.
The conventional analogue IC type brushless motor cannot read as information a speed of change in the power element temperature or a margin to the reference voltage to estimate an amount of change or a margin of the temperature in the future. Therefore, the protection operation temperature is generally determined, on the premise that the brushless motor performs a continuous operation, such that the junction temperature of the power element does not exceed Tjmax, and the maximum current value allowable at the temperature is set as the current limit value Ip1.
For this reason, the current limit value cannot be set equal to or greater than the current limit value Ip1. Therefore, the control has such a passive function that only the current limit value is lowered when the temperature of the power element reaches the protection operation temperature.